<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
  <script src="https://cdn.bootcdn.net/ajax/libs/vue/2.6.10/vue.js"></script>
</head>
<body>
<div id="app">
    <input type="text" v-model="msg"/>
    <button @click="count++">累加count</button>
    doubleCount:{{doubleCount}}
</div>
<script>
  const vm = new Vue({
    el:"#app",
    data(){
      return{
        msg:"",
        count:2
      }
    },
    computed:{
        doubleCount(){
            return this.count *2;
        }
    },
    watch:{
      msg(newVal,oldVal){
          console.log("发送请求","新值"+ newVal,"旧值"+ oldVal )
      },
      doubleCount() {
          console.log("重新计算了")
      }
    }
  })
</script>
</body>
</html>